The operations which are used to form metal strip or sheet may cause crystallographic anisotropy. Such crystallographic anisotropy arises mainly from rolling and annealing. During a rolling operation, there is a tendency for the metal crystals to adopt a preferred orientation. In the recrystallisation which occurs in a subsequent annealing operation, there is a tendency for the metal crystals to adopt another preferred orientation. Such crystallographic anisotropy leads to anisotropy in the stress-strain relationships in the metal strip or sheet. When a blank cut from metal strip or sheet is subjected to forming operations, such as drawing, wall ironing or pressing, strain variations lead to the formation of ears and valleys between the ears.
There will now be described three forming processes used in the manufacture of metal cans, each of which results in a workpiece exhibiting ears.
In the first process, a metal can body is formed from a circular blank cut from metal strip by subjecting the blank to a drawing operation followed by one or more redrawing operations. An example of the shape of a typical can body 10 following a second redrawing operation is shown in FIGS. 1 and 2. As may be seen, the can body has a seaming flange 12 and the flange 12 has four ears 14 which are caused by the anisotropy of the metal strip. Between each pair of adjacent ears 14, there is a valley. As the presence of the ears 14 would prevent the formation of a satisfactory seam with a can cover, the seaming flange 12 is trimmed back to the shape indicated by circular line 16. Consequently, the presence of ears 14 creates the need to perform a trimming operation and results in the wastage of the material removed in the trimming operation.
The number of ears exhibited by a can body after a second redrawing operation depends partly upon the nature of the operation used to form the metal sheet from which the blank is cut, and partly upon the type of metal used. Three common patterns are illustrated in FIGS. 3a,3b,4a,4b,5a and 5b. In FIGS. 3a,4a,5a, there are shown circular blanks 20,22,24 cut from sheet metal 26,28,30. In each of these figures, the rolling direction used to form the sheet metal is indicated by an arrow R and the directions in which the ears are formed are indicated by arrows E. Plan views of can bodies 32,34,36 formed, respectively, from blanks 20,22,24 are shown in FIGS. 3b,4b,5b. In these figures, ears are indicated by reference numeral 38.
In the example shown in FIGS. 3a,3b, ears are formed at 45.degree., 135.degree.,225.degree. and 315.degree. relative to the rolling direction. In the example shown in FIGS. 4a, 4b, ears are formed at 0.degree., 90.degree., 180.degree. and 270.degree. relative to the rolling direction. In the example shown in FIGS. 5a,5b, ears are formed at 0.degree., 60.degree., 120.degree., 180.degree., 240.degree. and 300.degree. relative to the rolling direction. In each example, there is a valley between each pair of adjacent ears.
In the second process used in the manufacture of metal cans, a metal can body is formed from a metal blank by a drawing operation, a redrawing operation and a wall ironing operation. In each of the drawing and redrawing operations, the workpiece is driven by a punch through a die and then removed from the punch by a stripper. In the wall ironing operation, the workpiece is driven by a punch through one or more wall ironing dies and then removed by a stripper. A perspective view of a can body 40 having ears 42 after a wall ironing operation is shown in FIG. 6. Between each pair of adjacent ears 42, there is a valley. The ears 42 have to be removed by a trimming operation and this causes wastage of material. After the wall ironing operation, the ears tend to interfere with normal operation of the stripper and such interference can cause the wall of the can body to buckle.
In the third process, a can cover is formed from a circular metal blank by a drawing operation and one or more redrawing operations. In FIG. 7, there is shown the peripheral part of a typical cover 44. The cover 44 includes a chuck wall 46, a seaming panel 48 and a cover curl 50. Ears are normally present in the free ends of the cover curl 50 and it is not usually possible to remove the ears with a trimming operation. In order to connect the cover 44 to a can body, the cover 44 is placed on the free end of a can body. The seaming panel 48 of the cover 44 and the seaming flange of the can body are then interlocked in a first seaming operation. The seaming panel and seaming flange are then squeezed together in a second seaming operation to form a double seam.
A typical double seam 52 is shown in FIG. 8. The double seam 52 includes a cover hook 54 and a body hook 56 having an overlap 58. The integrity of the double seam depends upon the length of this overlap 58. The presence of ears in the cover curl causes a variation in the length of the overlap 58. Usually, the sizes of the seaming panel and flange are sufficient to ensure that the minimum length of the overlap 58 is adequate to achieve a double seam of high integrity. However, for some applications, there is a requirement to make the dimensions of the double seam as small as possible. The presence of ears in the cover curl places a restriction on the minimum dimensions that may be achieved.
From the foregoing, it may be appreciated that the formation of ears leads to many problems.
One method of compensating for the formation of ears and valleys is to use a metal blank which is not quite round, but has a number of lobes, for example four, six or eight as may be appropriate, aligned to cancel the ear and valley forming properties of the metal sheet. The lobes of the blank fill the valleys between the ears.
In a known method of cutting lobed blanks, there are used a matched punch and die which have been ground to a lobed shape. This method suffers from the disadvantages that the punch and die are difficult to produce and it is difficult and time consuming to set the punch and die in the blanking apparatus as the lobes of the punch and die must be accurately aligned with each other.
It is an object of this invention to provide a new or improved apparatus for, and a new or improved method of, cutting a blank from metal strip of sheet.
According to one aspect of this invention, there is provided an apparatus for cutting a blank from metal strip or sheet, said apparatus comprising a punch having a cutting edge and a die having a cutting edge which is arranged to cooperate with the cutting edge of the punch, in which a plurality of circumferentially extending lobe-forming sections are provided in the cutting edge of said punch, each lobe-forming section being constructed by forming a recess in the cutting edge of said punch, said punch and die being arranged to cooperate to produce a blank having lobes at positions corresponding to the positions of said lobe-forming sections.
When making the punch and die for the apparatus of this invention, the only additional step that is required in comparison with the manufacture of a conventional punch and die is the formation of recesses in the cutting edge of the punch. When using the apparatus, the punch and die can be used in existing machines. When a blank is cut, the recesses in the cutting edge of the punch causes lobes to be formed.
According to another aspect of this invention, there is provided a method of cutting a blank from metal strip or sheet comprising the steps of taking a punch having a cutting edge and a die having a cutting edge which is arranged to cooperate with the cutting edge of the punch, a plurality of circumferentially extending lobe-forming sections being provided in the cutting edge of said punch and each lobe-forming section being constructed by forming a recess in the cutting edge of said punch, placing the metal strip or sheet between the punch and the die, and causing the punch and the die to cooperate so as to cut a blank from the metal strip or sheet, said blank having lobes at positions corresponding to the positions of said lobe-forming sections.